The present invention relates to a bushing used for a lead port of gas-insulated electrical equipment of a power substation of the construction in which the equipment is accommodated in a sealed container that is filled with an insulating gas.
A gas bushing is usually used for the lead port of gas-insulated electrical equipment. The interior of the gas bushing is filled with an insulating gas at a high pressure which is equal to the pressure in the gas-insulated electrical equipment, while a high tension is applied to the central conductor. If a material impinges upon a porcelain tube in the gas bushing that is used under the above-mentioned condition, or if flashover develops along the contaminated surface, or if electric insulation breaks down in the bushing, the porcelain tube may be destroyed and the high pressure insulating gas filling the container may discharge at once. Therefore, fractured pieces of the porcelain tube scatter around and come into collision with the neighboring equipment, giving rise to the occurrence of chain-like destruction phenomenon which may result in the destruction of the whole substation. In particular, in the 500-kV systems and in the 1000-kV systems that will be put into operation in the future, bushings having large volumes will be used, and the insulating gas will have increased compression energy. Therefore, destruction of a porcelain tube could result in tremendous secondary damage. In the 66- to 275-kV systems, the volume in the bushing will be small depending upon the class of voltage, and the compression energy will become small correspondingly. However, the distance of the bushing from the neighboring equipment will decrease too depending upon the class of voltage. It can therefore be considered that the chain-like destruction may take place when a porcelain tube is destroyed, irrespective of the class of voltage.
In order to minimize the secondary damage that may result when a porcelain tube of the gas bushing is destroyed, the inventor of the present invention has previously proposed in U.S. application Ser. No. 322,665 filed Nov. 18, 1981, entitled "Bushing for Gas-Insulated Electrical Equipment" the construction in which an insulation cylinder is provided in the porcelain tube to reduce the volume that will be discharged to the open air when the porcelain tube is destroyed in order to limit the discharge of energy, in an attempt to reduce the compression energy that will be emitted as soon as the porcelain tube is destroyed.
That is, as shown in FIG. 1, an insulation cylinder 5 is provided in a porcelain tube 2 so that the clearance 6 is minimized, the porcelain tube 2 having a central conductor 1 disposed at the center thereof, and metal flanges 3, 4 secured to the upper end and to the lower end thereof with cement. The upper end of the central conductor 1 and the upper metal flange 3 are hermetically sealed with a cover 7 at the upper end of the porcelain tube 2. An electrode 8 is provided in a mounting flange 9 to reduce the concentration of electric field, and is fastened onto a bushing-mounting portion 10. With this setup, the compression energy of gas contained in space of clearance 6 only will be discharged in case the porcelain tube 2 is destroyed by some cause. Therefore, the energy is not so great as to drive the fractured pieces of porcelain tube into distance. This may not give rise to the occurrence of chain-like destruction of equipment in the substation. The compression energy can be further reduced if the pressure in the clearance 6 is reduced. With the insulation cylinder 5 being inserted, however, the area for inserting the electrode 8 is narrowed compared with when there is no insulation cylinder 5. To uniformize the electric field in the porcelain tube, therefore, it becomes necessary to increase the diameter of the porcelain tube 2 to provide increased space for inserting the insulation cylinder 5.